Process for preparing flexible polyurethane foams

ABSTRACT

Process for preparing a flexible foam by reacting an isocyanate prepolymer having a free NCO content of 2-15% by weight, an isocyanate prepolymer having a free content of 25-31% by weight and an isocyanate-reactive composition comprising water.

This invention relates to a process for preparing flexible polyurethanefoams, reaction systems for preparing such foams and polyisocyanatecompositions useful for preparing such foams.

The preparation of flexible polymeric polyurethane foams is widelyknown.

In EP 392788 a method has been described using an isocyanate prepolymerhaving a free NCO content of from 2 to 15% by weight and anotherpolyisocyanate. The other polyisocyanate preferably is a diphenylmethanediisocyanate like MDI isomer mixtures, MDI variants and crude orpolymeric MDI.

Surprisingly it has been found tbat the use of the above prepolymertogether with another prepolymer gives improved processing and a reducednumber of closed cells and foam defects, even without using surfactant.In addition to that improvements regarding resilience and hysteris havebeen observed.

Therefore the present invention is concerned with a method for thepreparation of a flexible polyurethane foam by reacting an isocyanateprepolymer having a free NCO content of from 2 to 15% by weight whichprepolymer has been made from a polyoxyalkylene polyol which has anethylene oxide content of up to 30% by weight and another isocyanateprepolymer having a free NCO content of 25-31% by weight whichprepolymer has been made from a polyoxyalkylene polyol which has anethylene oxide content of at least 50% by weight, the weight ratio ofthe first and the second prepolymer ranging from 1.5-19:1, with anisocyanate-reactive composition comprising at least 40% by weight ofwater. Isocyanate-terminated prepolymer in this context is defined asthe reaction product of excess polyisocyanate and polyol including theunreacted polyisocyanate and any polyisocyanate which is added after thereaction took place.

The isocyanate-terminated prepolymer having a free NCO content in therange from 2 to 15% by weight is obtained by reacting a polyoxyalkylenepolyol having an average nominal functionality of 2 to 8, a numberaverage equivalent weight in the range from 500 to 5000 and an ethyleneoxide content of up to 30% by weight with a stoichiometric excess of apolyisocyanate and in particular of a diphenylmethane diisocyanatecomposition. This prepolymer will be referred to as "the firstprepolymer" hereinafter.

Diphenylmethane diisocyanate compositions which may be used in thepreparation of this first prepolymer include "pure" MDI preferablycontaining at least 55% by weight of the 4,4'-isomer. Suitableisocyanates therefore include the substantially pure 4,4'-isomer andisomer mixtures containing not more than 40%, preferably not more than30%, and more preferably not more than 20% by weight of the 2,4'-isomerand not more than 5% by weight of the 2,2'-isomer. Other suitablediphenylmethane diisocyanate compositions include modified forms ofthese diphenylmethane diisocyanates, that is to say MDI containing atleast 60% by weight of the 4,4'-isomer modified in known manner by theintroduction of urethane, allophanate, urea, bioret, carbodiimide,uretonimine or isocyanurate residues. These so-called MDI variantsparticularly include uretonimine-modified MDI having NCO contents of atleast 25% by weight and polyether-based prepolymers having NCO contentsof at least 20% by weight. Diphenylmethane diisocyanate compositionscontaining pure MDI and polymeric fillers may also be used in thepreparation of the first prepolymer. MDI compositions containingpolymeric fillers have been described in the prior art and includepolyurea dispersions in MDI and prepolymers based on MDI and polymerpolyols containing dispersed polymer particles having an NCO content ofat least 20% by weight. In these products, it is generally preferredthat the dispersed polymer particles have an average particle size ofless than 50 microns.

Further diphenylmethane diisocyanate compositions which may be used inthe preparation of the prepolymer include compositions containingpolymethylene polyphenylene polyisocyanates. Thus, mixtures may be usedcontaining at least 70% by weight of pure MDI and up to 30% by weight ofthe so-called crude MDI containing from 35 to 65% by weight ofdiisocyanates, the remainder being largely polymethylene polyphenylenepolyisocyanates having isocyanate functionalities greater than 2.Mixtures may also be used of pure MDI and polymeric MDI compositionscontaining higher proportions (up to 100%) of the said higherfunctionality polyisocyanates. Still further diphenylmethanediisocyanate compositions which may be used in preparing the firstprepolymer include mixtures of the above described MDI types and up to20% by weight of other polyisocyanates. Other polyisocyanates which maybe used in admixture with the MDI include aliphatic, cycloaliphatic andaraliphatic polyisocyanates, especially diisocyanates, for examplehexamethylene diisocyanate, isophorone diisocyanate,cyclohexane-1,4-diisocyanate, 4,4'-dicyclohexylmethane diisocyanate andm- and p-tetramethylxylene diisocyanates and, especially, aromaticpolyisocyanates such as tolylene and phenylene diisocyanates andmixtures thereof.

Whereas a preferred embodiment of the invention involves the use ofmethylene-bridged polyphenyl polyisocyanates as disclosed hereinabove,the first prepolymer may also be made from other polyisocyanates, suchas in particular aliphatic, cycloaliphatic and araliphaticpolyisocyanates, especially diisocyanates, for example hexamethylenediisocyanate, isophorone diisocyanate, cyclohexane-1,4-diisocyanate,4,4'-dicyclohexyl-methane diisocyanate and m- and p-tetramethylxylenediisocyanates and, especially, aromatic polyisocyanates such as tolyleneand phenylene diisocyanates.

Preferred tolylene diisocyanate compositions for use in this embodimentof the invention are the so-called TDI 80/20 (a 80:20 mixture of2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate), TDI 65/35 (a65:35 mixture of 2,4-tolylenediisocyanate and 2,6-tolylenediisocyanate),and high --NCO prepolymers thereof.

The polyols used in the preparation of the first prepolymer have anaverage nominal hydroxyl functionality of 2 to 8. The term "nominalfunctionality" refers to the functionality, with respect to isocyanates,a polyol would be expected to have having regard to its monomericcomponents. For example, a polyether prepared by the addition ofpropylene oxide to a glycol will have a nominal hydroxyl functionalityof 2 although, in fact, its average functionality will be somewhat lessthan 2.

Thus, for a polyether polyol, the average nominal functionality is theaverage functionality (number of active hydrogen atoms) of the initiatoror initiators used in its preparation.

The average nominal functionality of the polyols preferably is 2 to 4,more preferably 2 to 3. Number average equivalent weights of the polyolsused are in the range from 750 to 5000, preferably in the range from1000 to 4000. The ethylene oxide content of the polyols used is up to30% by weight calculated on the total weight of the polyol. Mixtures oftwo or more polyols varying in functionality, equivalent weight and/orchemical constitution may be used provided such mixtures conform to theaverage functionality, average equivalent weight and ethylene oxidecontent criteria specified herein. Polyoxyalkylene polyols which may beused include products obtained by the polymerisation of a cyclic oxide,for example ethylene oxide, propylene oxide, butylene oxide ortetrahydrofuran in the presence, where necessary, of polyfunctionalinitiators. Suitable initiator compounds contain a plurality of activehydrogen atoms and include water and polyols, for example ethyleneglycol, propylene glycol, diethylene glycol, cyclohexane dimethanol,resorcinol, bisphenol A, glycerol, trimethylolpropane, 1,2,6-hexanetriolor pentaerythritol. Mixtures of initiators and/or cyclic oxides may beused.

Further particularly useful polyether polyols include polyoxypropylenediols and triols obtained by the simultaneous or sequential addition ofethylene and propylene oxides to di- or trifunctional initiators asfully described in the prior art provided the ethylene oxide content isnot higher than 30% by weight. Random copolymers, block copolymers andrandom/block copolymers having oxyethylene contents of up to 30%, basedon the total weight of the polyol may be used. Mixtures of the saiddiols and triols can be particularly useful. Other particularly usefulpolyether polyols include polytetra-methylene glycols obtained by thepolymerisation of tetrahydrofuran. Particularly useful are also mixturesof polypropylene oxide - polyethylene oxide polyols with up to 5% ofanother polyol, for example a polyalkylene oxide, a polyester polyol, apolycarbonate polyol, a polyacetal polyol or a polytetramethyleneglycol. Other polyols which may be used comprise dispersions orsolutions of addition or condensation polymers in polyols of the typesdescribed above. Such modified polyols have been fully described in theprior art and include products obtained by the in situ polymerisation ofone or more vinyl monomers, for example styrene and acrylonitrile, inpolyether polyols, or by the in-situ reaction between a polyisocyanateand an amino- and/or hydroxy-functional compound, such astriethanolamine, in a polyol.

The first isocyanate-terminated prepolymer may be prepared by reactingthe polyisocyanate with the polyol under conditions that have been fullydescribed in the prior art for the preparation of prepolymers. Reactiontemperatures of 40° to 90° C. are generally suitable for the preparationof the prepolymers. To achieve a final NCO content within the range 2 to15% by weight, an initial ratio of isocyanate to hydroxyl groups wouldtypically be within the range from 3:1 to 20:1. Preferred prepolymersare made by reacting the starting materials at an initial ratio ofisocyanate to hydroxyl groups in the range from 3.5:1 to 15:1,especially 4:1 to 10:1, to give prepolymers having free NCO contents of4 to 12%, especially of 5 to 10% by weight. After the reaction tookplace one or more of the polyisocyanates mentioned before may be addedto the reaction product. The amount of the polyisocyanate added shouldbe such that the prepolymer still has an NCO content of 2 to 15 % byweight. In addition to the first prepolymer, another prepolymer is usedin the process according to the invention.

The prepolymer having a free NCO content of 25-31% by weight,hereinafter referred to as the second prepolymer is made from apolymethylene polyphenylene polyisocyanate having an isocyanatefunctionality of 2.5-3.0 and a polyoxyalkylene polyol having a nominalhydroxyl functionality of 2-6 and a number average equivalent weight offrom 250 to 3000. The polyol should have an ethylene oxide content of atleast 50% and preferably of 60 to 90% by weight calculated on the wholepolyol. The polyisocyanates and polyols may be selected from thosementioned hereinbefore for preparing the first prepolymer provided thepolyisocyanates and polyols conform to the above restrictions. Thesecond prepolymer is prepared in a similar way as the first one with theproviso that the NCO:OH ratio in general is higher in order to obtain aprepolymer having a free NCO content of 25-31% by weight. After thereaction took place one or more of the polyisocyanates mentioned beforemay be added provided the NCO content remains 25-31% by weight.

In order to prepare the foam, preferably 100 parts by weight of apolyisocyanate composition comprising 60-95% by weight of the firstprepolymer and 5-40% by weight of the second prepolymer calculated onthe total weight of the first and the second prepolymer is reacted with1 to 20 parts by weight of the isocyanate-reactive composition in thepresence as necessary of conventional additives which may forconvenience be included in the isocyanate-reactive composition or, ifinert towards isocyanates, in the polyisocyanate or as an additionalstream.

In many cases, water will be the sole isocyanate-reactive speciespresent in the isocyanate-reactive composition. In addition tocontaining water, however, the isocyanate-reactive composition may alsocontain up to 60% by weight of other isocyanate-reactive compounds, inparticular those having an average equivalent weight below 500.Preferably the isocyanate-reactive composition comprises at least 50% byweight of water, and for certain applications at least 95% by weight.Further the isocyanate-reactive composition may comprise a minor amountof a higher molecular weight polyol.

Suitable isocyanate reactive compounds having an average equivalentweight below 500 are polyfunctional isocyanate-reactive aliphatic,cycloaliphatic, aromatic or araliphatic compounds or mixtures thereoflike ethylene glycol, propylene glycol, dipropylene glycol, 1,4-butanediol, sorbitol, monoethanolamine, diethanolamine, 1,2-diaminoethane,amine-terminated polyethers of molecular weight below 1000, isophoronediamine (1-amino-3-amino-3,5,5-trimethyl cyclohexane), fullyhydrogenated di(aminophenyl)methane, piperazine, hydrogenated methylenedianiline, diamino menthane and hydrogenated toluene diamine, diethyltoluene diamine and Mannich derivatives of phenols or alkyl phenols,particularly Mannich condensates of phenol or phenol derivatives,formaldehyde and diethanolamine like 2,5 bis(N,N-diethanolamine)-4-nonylphenol.

Additives which may be used include the catalysts commonly disclosed inthe literature, such as for example tertiary amines and tin compounds,surface-active agents and foam stabilisers, for examplesiloxane-oxyalkylene copolymers, flame retardants, organic and inorganicfillers, pigments and internal mould release agents. Preferably theprocess according to the invention is conducted in the absence of achlorofluorocarbon blowing agent like trichlorofluoromethane.

The reaction for preparing the flexible foams according to the presentinvention in general is conducted at an index of from 35 to 125preferably of from 45 to 105; the index being defined by the formula##EQU1## The present invention is further concerned with apolyisocyanate composition comprising 60-95% by weight of an isocyanateprepolymer having a free NCO content of from 2 to 15% by weight whichprepolymer has been made from a polyoxyalkylene polyol which has anethylene oxide content of up to 30% by weight and 5-40% by weight ofanother isocyanate prepolymer having a free NCO content of 25-31% byweight which prepolymer has been made from a polyoxyalkylene polyolwhich has an ethylene oxide content of at least 50% by weight, the % byweight of the first and the second prepolymer being calculated on thetotal weight of the first and the second prepolymer.

As is known producers of polyisocyanates, isocyanate-terminatedprepolymers and polyols are often not producing the foams; they sell theabove chemicals to a foam producer. Often a fully formulated reactionsystem is sold to a foam producer; i.e. a polyisocyanate composition anda polyol composition comprising all other ingredients, like blowingagent, catalysts etc, are sold as a reaction system in separatecontainers in order to allow the customer to make a specific foam byreacting the two compositions. This approach provides a relativelysimple way to the customer to make a product from often a variety ofchemicals by combining two compositions under appropriate conditions. Adisadvantage of this approach is that the foam producer is limited inadapting the reaction system to his specific needs. Therefore it isconsidered advantageous in the context of the present invention not toprovide a mixture of the first and the second prepolymer but rather toprovide the first and the second prepolymer separately in a reactionsystem also comprising the isocyanate-reactive composition. Consequentlythe invention is concerned with a reaction system comprising (A) anisocyanate prepolymer having a free NCO content of from 2 to 15% byweight which prepolymer has been made from a polyoxyalkylene polyolwhich has an ethylene oxide content of up to 30% by weight and (B)another isocyanate prepolymer having a free NCO content of 25-31% byweight which prepolymer has been made from a polyoxyalkylene polyolwhich has an ethylene oxide content of at least 50% by weight, theweight ratio of the first and the second prepolymer ranging from1.5-19:1 and (C) an isocyanate-reactive composition comprising at least40% by weight of water. Preferably the reaction system comprises 60-95%by weight of the first prepolymer, 5-40% by weight of the secondprepolymer, calculated on the total weight of the first and the secondprepolymer, and 1-20 parts by weight of the isocyanate-reactivecomposition per 100 parts by weight of first and second prepolymer.

The term "reaction system" is defined as a combination of ingredientsfor preparing the flexible foams according to the present inventionwhich ingredients are kept in separate containers and which ingredientsare sold and marketed for preparing said flexible foams. Hence theingredients (A), (B) and (C) are kept separate in the above reactionsystem.

The reaction system with the separate prepolymers provides the foamproducer with the opportunity to supply the prepolymers independentlyfrom each other to the isocyanate-reactive composition. Therefore apreferred embodiment of the process according to the present inventionis to bring the first and the second prepolymer independently from eachother into contact with the isocyanate-reactive composition. In generalthis is conducted by providing a conducting device, like a pipe, from acontainer containing the first prepolymer to a mixing head or a reactionzone of a device for making foams and a second conducting device from acontainer containing the second prepolymer to the said mixing head orreaction zone and a third conducting device from a container containingthe isocyanate-reactive composition to the said mixing head or reactionzone. The important feature of the embodiment is providing the twoprepolymers from separate containers giving the foam producer theopportunity to adapt their relative amounts. It is within thisembodiment to combine the first two conducting devices to one singleconducting device just before or at the reaction zone or the mixinghead. It is also within this embodiment to provide the first and thesecond conducting device from the respective containers to a mixing zonewherein the two prepolymers are mixed and to provide a conducting devicefrom this mixing zone to the said reaction zone or mixing head whereinthe prepolymer mixture is brought into contact with theisocyanate-reactive composition.

It will be clear to those skilled in the art that likewise theisocyanate-reactive ingredients and/or the other polyisocyanatesmentioned before could be provided to the reaction zone or the mixinghead via more than one conducting device. For certain applications thisapproach may be preferred in particular for preparing slab-stockflexible foams.

In carrying out the invention it is most appropriate to form ahomogeneous liquid mixture of the prepolymers prior to their reactionwith the isocyanate-reactive composition.

The homogeneous liquid mixture is created by metering specific amountsof the prepolymers into a mixing zone at controlled rates to produce thedesired weight ratio of prepolymers. Preferably this mixing zoneinvolves high shear mixing or impingement mixing. Each prepolymer is fedinto the mixing zone as a separate metered stream using appropriateliquid metering pumps from a storage tank. If necessary, the storagetanks and pumping systems may be heated such that the prepolymersentering the mixing zone are at the desired viscosity and temperature,after which mixing they may be fed into a reaction zone to react withthe isocyanate-reactive composition to form the desired foam.

The mixing zone can suitably be an appropriately shaped container havinghigh shear mixing. The container can suitably be a closed cylinderhaving a multiplicity of entrance ports which are preferablydiametrically opposed such that the advantage of impingement mixing isrealised in addition to the mixing forces applied by high shear mixingblades, rotating therein.

High shear mixing forces are usually provided by appropriately shapedmixing blades rotating at high speeds and located at the centre of themixing zone. In addition to the isocyanate other additives may beincorporated at the mixing zone such as catalysts, surfactants,colouring agents, stabilisers and the like.

After mixing for a finite period of time to form a homogeneous mix, theisocyanate blend may be temporarily stored in a holding zone ortransferred instantly to a reaction zone.

The reaction zone may be an adjacent portion of the mixing container ora separate container. A preferred mixing device comprises an elongatedcylindrical container having high shear mixing blades mounted on arotating shaft passing the length of the cylinder. The diameter andlength of the cylindrical mixing device is sized appropriately to matchthe feed rate, holdup time and cream time required. Multiple feed portsare positioned at one end of the container to receive unmixedisocyanates and other additives while an exit port is located at theopposite end of the container to emit mixture.

The method of the invention may be used to produce moulded or slabstockfoam continuously, semi-continuously or badge-wise.

The invention is illustrated but not limited by the following examplesin which all parts and percentages are by weight unless otherwisestated.

The following glossary of materials is included to identify reactioncomponents not otherwise identified in the examples.

    ______________________________________                                        Glossary                                                                      ______________________________________                                        Prepolymer 1                                                                            The reaction product of a polyol having a                                     functionality of 3, an ethylene oxide content of                              about 14% by weight (tip) and a molecular                                     weight of about 4500 and pure MDI having a                                    2,4' isomer content of 10% by weight, the                                     prepolymer having an NCO content of 6.5%                                      by weight.                                                          Polymeric MDI                                                                           Polymeric MDI having an NCO content of 30.7                                   and a functionality of 2.7 and a diisocyanate                                 content of 42% by weight.                                           Prepolymer 2a                                                                           The reaction product of the above polymeric                                   MDI with 4% by weight of a polyol having a                                    molecular weight of about 4000, a functionality                               of 3 and an ethylene oxide content of 75% by                                  weight; the ethylene oxide being randomly                                     distributed. The NCO content is 29.4% by                                      weight.                                                             Prepolymer 2b                                                                           As prepolymer 2a with the proviso that 6% by                                  weight of the polyol is used. The NCO content is                              28.7% by weight.                                                    Prepolymer 2c                                                                           As prepolymer 2a with the proviso that 10% by                                 weight of the polyol is used. The NCO content is                              27.4% by weight.                                                    Polyol    A polyol having a molecular weight of about                                   4000, a functionality of 3 and an ethylene oxide                              content of 75% by weight; the ethylene oxide                                  being randomly distributed.                                         Niax A1   Catalyst from Union Carbide.                                        SH 210    Silicon surfactant from Union Carbide.                              D33 LV    Catalyst from Air Products.                                         DMI       1,2-dimethyl-imidazole                                              ______________________________________                                    

Example

Flexible foams were prepared by combining the ingredients given inTable 1. The physical properties of the foams obtained are given inTable 1 as well.

                  TABLE 1                                                         ______________________________________                                        Experiment                                                                              1        2        3     4    5    6                                 ______________________________________                                        Prepolymer 1                                                                            80       80       80    80   86.66                                                                              86.66                             Polymeric MDI                                                                           20       --       --    --   13.44                                                                              --                                Prepolymer 2a                                                                           --       20       --    --   --   13.44                             Prepolymer 2b                                                                           --       --       20    --   --   --                                Prepolymer 2c                                                                           --       --       --    20   --   --                                Polyol    --       --       --    --   0.5  --                                H.sub.2 O 4.6      4.6      4.6   4.6   3.32                                                                               3.32                             Niax A1   0.2      0.2      0.2   0.2  --   --                                SH 210    --       --       --    --   0.4  --                                D33 LV    0.4      0.4      0.4   0.4  --   --                                DMI/H.sub.2 O                                                                           --       --       --    --   0.6  0.6                               80/20                                                                         Index     52       51       50    49   60   60                                Free rise 24       25       25    26   36   35                                density, Kg/m.sup.3                                                           Open/closed                                                                             very     closed;  ±open                                                                            open open open                              cell content                                                                            closed,  easily                                                               skrinked crushable                                                  Resilience, %                                                                           --       --       --    --   51   54                                ______________________________________                                    

The foam according to experiment 6 which is according to the presentinvention did not show surface defects although no surfactant was usedwhile the foam according to experiment 5 which is a comparative exampleshowed some surface defects despite the fact that a surfactant wasemployed. In experiments 1-% the foams were made by first mixing thepolyisocyanates with each other and by mixing the water with thecatalysts and then by combining the two mixtures in an open containerwherein they were allowed to react.

In experiments 5 and 6 the ingredients used were fed independently fromcommercially available equipment including individual holding tanks eachequipped with a metering pump and preheater into a mixing devicemanufactured by Viking PTI Ltd as Model No. TC 1167 according to theproportions as outlined in Table 1. The elongated cylindrical mixinghead employed has an I.D. of 10 cms, and a length of 50 cms withcentrally mounted rotating shaft having sixty two 0.8 cm O.D. high shearmixing blades mounted 2 centimeters apart along the length of the 40centimeter shaft. Entrance ports are positioned 2,5 cms from the top ofthe mixing end of the container. Additives are fed in to the mixing zonethrough feed ports located 13 cms below the prepolymer and isocyanatefeed ports. A cream exit port of 5 cms I.D. is centrally located at thebottom of the cylinder. The water feed port is positioned in the sidewall at a position 15 cms below the isocyanate feed ports. In operationwith a shaft rotating speed of 3000 RPM, the total feed weight of theingredients entering the mixing device ranges from 100-150 kg/min. toadjust a holdup time in the isocyanate mixing zone at 400 to 269microseconds prior to entering the formation zone for the creamingmixture, where the holdup time is 1.5 to 0.8 seconds. The creamingmixture is fed from the mixing device to a creaming and/or foaming zoneto produce flexible slabstock foam.

We claim:
 1. Process for preparing a flexible polyurethane foam byreacting an isocyanate prepolymer having a free NCO content of from 2 to15% by weight which prepolymer has been made from a polyoxyalkylenepolyol which has an ethylene oxide content of up to 30% by weight andanother isocyanate prepolymer having a free NCO content of 25-31% byweight which prepolymer has been made from a polyoxyalkylene polyolwhich has an ethylene oxide content of at least 50% by weight, theweight ratio of the first and the second prepolymer ranging from1.5-19:1, with an isocyanate-reactive composition comprising at least40% by weight of water.
 2. Process according to claim 1 characterised inthat 100 parts by weight of a polyisocyanate composition comprising60-95% by weight of the first prepolymer and 5-40% by weight of thesecond prepolymer calculated on the total weight of the first and thesecond prepolymer is reacted with 1-20 parts by weight of theisocyanate-reactive composition.
 3. Process according to claim 1characterised in that the first and the second prepolymer are broughtinto contact with the isocyanate-reactive composition independently fromeach other.
 4. Process according to claim 1 characterised in that thesecond prepolymer is made from a polymethylene polyphenylenepolyisocyanate having an isocyanate functionality of 2.5-3.0 and apolyoxyalkylene polyol having an average nominal hydroxyl functionalityof 2-6 and a number average equivalent weight of from 250 to
 3000. 5.Process according to claim 1 characterised in that 100 parts by weightof a polyisocyanate composition comprising 60-95% by weight of the firstprepolymer and 5-40% by weight of the second prepolymer calculated onthe total weight of the first and the second prepolymer is reacted with1-20 parts by weight of the isocyanate-reactive composition and that thefirst and the second prepolymer are brought into contact with theisocyanate-reactive composition independently from each other and thatthe second prepolymer is made from a polymethylene polyphenylenepolyisocyanate having an isocyanate functionality of 2.5-3.0 and apolyoxyalkylene polyol having an average nominal hydroxyl functionalityof 2-6 and a number average equivalent weight of from 250 to
 3000. 6.Reaction system comprising (A) an isocyanate prepolymer having a freeNCO content of from 2 to 15% by weight which prepolymer has been madefrom a polyoxyalkylene polyol which has an ethylene oxide content of upto 30% by weight and (B) another isocyanate prepolymer having a free NCOcontent of 25-31% by weight which prepolymer has been made from apolyoxyalkylene polyol which has an ethylene oxide content of at least50% by weight, the weight ratio of the first and the second prepolymerranging from 1.5-19:1 and (C) an isocyanate-reactive compositioncomprising at least 40% by weight of water.
 7. Reaction system accordingto claim 6 characterised in that the reaction system comprises 60-95% byweight of the first prepolymer, 5-40% by weight of the secondprepolymer, calculated on the total weight of the first and the secondprepolymer, and 1-20 parts by weight of the isocyanate-reactivecomposition per 100 parts by weight of first and second prepolymer. 8.Reaction system according to claim 6 characterised in that the secondprepolymer is made from a polymethylene polyphenylene polyisocyanatehaving an isocyanate functionality of 2.5-3.0 and a polyoxyalkylenepolyol having an average nominal hydroxyl functionality of 2-6 and anumber average equivalent weight of from 250 to
 3000. 9. Reaction systemaccording to claim 7 characterised in that the second prepolymer is madefrom a polymethylene polyphenylene polyisocyanate having an isocyanatefunctionality of 2.5 to 3.0 and a polyoxyalkylene polyol having anaverage nominal hydroxyl functionality of 2 to 6 and a number averageequivalent weight of from 250-3000.